High thermal conductivity ceramic body

ABSTRACT

A process for producing an aluminum nitride ceramic body having a composition defined and encompassed by polygon ABCDEFGH excluding line DE of FIG. 2, a porosity of less than about 10% by volume, and a thermal conductivity greater than 0.70 W/cm·K at 25° C. which comprises forming a mixture comprised of aluminum nitride powder containing oxygen, an Ln oxide selected from the group consisting essentially of erbium oxide, holmium oxide, a mixture and solid solution thereof, and free carbon, shaping said mixture into a compact, said mixture and compact having a composition wherein the equivalent % of Ln and aluminum ranges from point A up to point E of FIG. 2, said mixture and said compact having an equivalent % composition of Ln, Al, O and N outside the composition defined and encompassed by polygon ABCDEFGH of FIG. 2, heating said compact up to a temperature at which its pores remain open reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Ln, O and N is defined and encompassed by polygon ABCDEFGH excluding line DE of FIG. 2, and sintering said deoxidized compact producing said ceramic body.

The following patents and applications were filed in the names of IrvinCharles Huseby and Carl Francis Bobik and are assigned to the assigneehereof and are incorporated herein by reference:

U.S. Pat. Nos. 4,478,785 and 4,533,645 disclose a process comprisingforming a mixture of aluminum nitride powder and free carbon wherein thealuminum nitride has a predetermined oxygen content higher than about0.8% by weight and wherein the amount of free carbon reacts with suchoxygen content to produce a deoxidized powder or compact having acertain oxygen content, heating the mixture or a compact thereof toreact the carbon and oxygen producing the deoxidized aluminum nitride,and sintering a compact of the deoxidized aluminum nitride producing aceramic body having a thermal conductivity greater than 0.5 W/cm·K at22° C.

U.S. Pat. No. 4,547,471 discloses a process for producing a sinteredaluminum nitride ceramic body having a composition defined andencompassed by line ABCDEFA but not including lines CD and EF of FIG. 1therein and a thermal conductivity greater than 1.0 W/cm·K at 22° C.which comprises forming a mixture comprised of aluminum nitride powderand an yttrium additive selected from the group consisting of yttrium,yttrium hydride, yttrium nitride and mixtures thereof, said aluminumnitride and yttrium additive having a predetermined oxygen content, saidmixture having a composition wherein the equivalent % of yttrium,aluminum, nitrogen and oxygen is defined and encompassed by line ABCDEFbut not including lines CD and EF in FIG. 1, therein shaping saidmixture into a compact, and sintering said compact.

U.S. Pat. No. 4,578,234 discloses a process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon JKLM but not including line MJ of FIG. 4 therein which comprisesforming a mixture comprised of aluminum nitride powder containingoxygen, yttrium oxide, and free carbon, shaping said mixture into acompact, said mixture and said compact having a composition wherein theequivalent % of yttrium and aluminum ranges from point L to less thanpoint J of FIG. 4 therein, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon JKLM of FIG. 4 therein, heating said compact upto a temperature at which its pores remain open reacting said freecarbon with oxygen contained in said aluminum nitride producing adeoxidized compact, said deoxidized compact having a composition whereinthe equivalent % of Al, Y, O and N is defined and encompassed by polygonJKLM but not including line MJ of FIG. 4 therein, and sintering saiddeoxidized compact.

U.S. Pat. No. 4,578,233 discloses a process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon FJDSR but not including line RF of FIG. 4 therein whichcomprises forming a mixture comprised of aluminum nitride powdercontaining oxygen, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges from point D upto point F of FIG. 4 therein, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon FJDSR of FIG. 4 therein, heating said compact upto a temperature at which its pores remain open reacting said freecarbon with oxygen contained in said aluminum nitride producing adeoxidized compact, said deoxidized compact having a composition whereinthe equivalent % of Al, Y, O and N is defined and encompassed by polygonFJDSR but not including line RF of FIG. 4 therein, and sintering saiddeoxidized compact.

U.S. Pat. No. 4,578,365 discloses a process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon P1N1KJ but not including lines KJ and P1J of FIG. 4 thereinwhich comprises forming a mixture comprised of aluminum nitride powdercontaining oxygen, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges between points Kand P1 of FIG. 4 therein, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon P1N1KJ of FIG. 4 therein, heating said compact upto a temperature at which its pores remain open reacting said freecarbon with oxygen contained in said aluminum nitride producing adeoxidized compact, said deoxidized compact having a composition whereinthe equivalent % of Al, Y, O and N is defined and encompassed by polygonP1N1KJ but not including lines KJ and P1J of FIG. 4 therein, andsintering said deoxidized compact.

U.S. Pat. No. 4,578,364 discloses a process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon P1JFA4 but not including lines JF and A4F of FIG. 4 thereinwhich comprises forming a mixture comprised of aluminum nitride powdercontaining oxygen, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges between points Jand A4 of FIG. 4 therein, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon P1JFA4 of FIG. 4 therein, heating said compact toa temperature at which its pores remain open reacting said free carbonwith oxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonP1JFA4 but not including lines JF and A4F of FIG. 4 therein, andsintering said deoxidized compact.

U.S. Pat. No. 4,578,232 discloses a process for producing an aluminumnitride ceramic body having a composition defined and encompassed bypolygon LT1DM but not including lines LM and DM of FIG. 4 therein whichcomprises forming a mixture comprised of aluminum nitride powdercontaining oxygen, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges from point T1 upto point M of FIG. 4 therein, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon LT1DM of FIG. 4 therein, heating said compact toa temperature at which its pores remain open reacting said free carbonwith oxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonLT1DM but not including lines LM and DM of FIG. 4 therein, and sinteringsaid deoxidized compact.

Copending U.S. patent application Ser. No. 810,473 filed on Dec. 18,1985 discloses a process for producing an aluminum nitride ceramic bodyhaving a composition defined and encompassed by polygon A4F5F6F4 butexcluding line F6F4 of FIG. 4 therein which comprises forming a mixturecomprised of aluminum nitride powder containing oxygen, yttrium oxide,and free carbon, shaping said mixture into a compact, said mixture andsaid compact having a composition wherein the equivalent % of yttriumand aluminum ranges from point F5 up to point F4 of FIG. 4 therein saidcompact having an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon A4F5F6F4 of FIG. 4therein, heating said compact to a temperature at which its pores remainopen reacting said free carbon with oxygen contained in said aluminumnitride producing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon A4F5F6F4 but excluding line F6F4 of FIG. 4therein, and sintering said deoxidized compact.

The present invention relates to the production of a liquid phasesintered polycrystalline aluminum nitride body having a thermalconductivity higher than 0.70 W/cm·K at 25° C. In one aspect of thepresent process, aluminum nitride is deoxidized by carbon to a certainextent, and then it is further deoxidized and/or sintered by utilizingerbium oxide, holmium oxide or a mixture thereof to produce the presentceramic.

A suitably pure aluminum nitride single crystal, containing 300 ppmdissolved oxygen, has been measured to have a room temperature thermalconductivity of 2.8 W/cm·K, which is almost as high as that of BeOsingle crystal, which is 3.7 W/cm·K, and much higher than that of α-Al₂O₃ single crystal, which is 0.44 W/cm·K. The thermal conductivity of analuminum nitride single crystal is a strong function of dissolved oxygenand decreases with an increase in dissolved oxygen content. For example,the thermal conductivity of aluminum nitride single crystal having 0.8wt % dissolved oxygen, is about 0.8 W/cm·K.

Aluminum nitride powder has an affinity for oxygen, especially when itssurface is not covered by an oxide. The introduction of oxygen into thealuminum nitride lattice in aluminum nitride powder results in theformation of Al vacancies via the equation: ##EQU1## Thus, the insertionof 3 oxygen atoms on 3 nitrogen sites will form one vacancy on analuminum site. The presence of oxygen atoms on nitrogen sites willprobably have a negligible influence on the thermal conductivity of AlN.However, due to the large difference in mass between an aluminum atomand a vacancy, the presence of vacancies on aluminum sites has a stronginfluence on the thermal conductivity of AlN and, for all practicalpurposes, is probably responsible for all of the decrease in the thermalconductivity of AlN.

There are usually three different sources of oxygen in nominally pureAlN powder. Source #1 is discrete particles of Al₂ O₃. Source #2 is anoxide coating, perhaps as Al₂ O₃, coating the AlN powder particles.Source #3 is oxygen in solution in the AlN lattice. The amount of oxygenpresent in the AlN lattice in AlN powder will depend on the method ofpreparing the AlN powder. Additional oxygen can be introduced into theAlN lattice by heating the AlN powder at elevated temperatures.Measurements indicate that at ˜1900° C. the AlN lattice can dissolve˜1.2 wt % oxygen. In the present invention, by oxygen content of AlNpowder, it is meant to include oxygen present as sources #1, #2 and #3.Also, in the present invention, the oxygen present with AlN powder assources #1, #2 and #3 may be removed by utilizing free carbon, and theextent of the removal of oxygen by carbon depends largely on thecomposition desired in the resulting sintered body.

According to the present invention, aluminum nitride powder can beprocessed in air and still produce a ceramic body having a thermalconductivity greater than 0.70 W/cm·K at 25° C.

Erbium oxide, Er₂ O₃, and holmium oxide, Ho₂ O₃, are lanthanide (Ln)oxides. The terms lanthanide or Ln are used herein to denote erbium,holmium and a mixture and/or solid solution thereof. For example, LnAlO₃denotes ErAlO₃ or HoAlO₃ or ErAlO₃ +HoAlO₃ or Er_(1-x) Ho_(x) AlO₃(O<x<1).

In one embodiment of the present invention, the aluminum nitride in acompact comprised of particulate aluminum nitride of known oxygencontent, free carbon and Ln oxide, is deoxidized by carbon to produce adesired equivalent composition of Al, N, Ln and O, and the deoxidizedcompact is sintered by means of a liquid phase containing mostly Ln andO and a smaller amount of Al and N.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the figures accompanying and forming apart of the specification in which:

FIG. 1 is a composition diagram showing the subsolidus phase equilibriain the reciprocal ternary system comprised of AlN, LnN, Ln₂ O₃ and Al₂O₃. FIG. 1 is plotted in equivalent % and along each axis of ordinatesthe equivalent % of oxygen is shown (the equivalent % of nitrogen is100% minus the equivalent % of oxygen). Along the axis of abscissas, theequivalent % of Ln is shown (the equivalent % of aluminum is 100% minusthe equivalent % of Ln). In FIG. 1, polygon ABCDEFGH but excluding lineDE encompasses and defines the composition of the sintered body producedby the present process; and

FIG. 2 is an enlarged view of the section of FIG. 1 showing thecomposition of the present polycrystalline body.

FIGS. 1 and 2 were developed algebraically on the basis of experimentaldata which included experiments with non-erbium compounds. Although noexperiments were carried out with a holmium compound, holmium oxide isconsidered substantially equivalent to erbium oxide and will form thesame phases in the sintered body herein.

The best method to plot phase equilibria that involve oxynitrides andtwo different metal atoms, where the metal atoms do not change valence,is to plot the compositions as a reciprocal ternary system as is done inFIG. 1. In the particular system of FIG. 1 there are two types ofnon-metal atoms (oxygen and nitrogen) and there can be considered to betwo types of metal atoms (Ln and aluminum). The Al, Ln, oxygen andnitrogen are assumed to have a valence of +3, +3, -2, and -3,respectively. All of the Al, Ln, oxygen and nitrogen are assumed to bepresent as oxides, nitrides or oxynitrides, and to act as if they havethe aforementioned valences.

The phase diagrams of FIGS. 1 and 2 are plotted in equivalent percent.The number of equivalents of each of these elements is equal to thenumber of moles of the particular element multiplied by its valence.Along the ordinate is plotted the number of oxygen equivalentsmultiplied by 100% and divided by the sum of the oxygen equivalents andthe nitrogen equivalents. Along the abscissa is plotted the number of Lnequivalents multiplied by 100% and divided by the sum of the Lnequivalents and the aluminum equivalents. All compositions of FIGS. 1and 2 are plotted in this manner.

Compositions on the phase diagrams of FIGS. 1 and 2 can be used todetermine the weight percent and the volume percent of the variousphases. For example, a particular point in the polygon ABCDEFGH in FIG.2 can be used to determine the phase composition of the polycrystallinebody at that point.

FIGS. 1 and 2 show the composition and the phase equilibria of thepolycrystalline body in the solid state.

The phase equilibria for the AlN-ErN-Er₂ O₃ -Al₂ O₃ system will be verysimilar to the phase equilibria for the AlN-HoN-Ho₂ O₃ -Al₂ O₃ system.For example, the tie lines between AlN and Er₂ O₃, Er₄ Al₂ O₉, ErAlO₃and Er₃ Al₅ O₁₂ will be substantially the same as the tie lines betweenAlN and Ho₂ O₃, Ho₄ Al₂ O₉, HoAlO₃ and Ho₃ Al₅ O₁₂. As a result, Ln canbe used to denote Er, or Ho, or Er+Ho or Er_(1-x) Ho_(x) where O<x<1.

For the case of Ln denoting Er_(1-x) Ho_(x), i.e. a solid solution, theequilibrium region between AlN and Ln₂ O₃ would denote the equilibriumregion between AlN and Er_(2-2x) Ho_(2x) O₃. The equilibrium regionbetween AlN, Ln₄ Al₂ O₉ and LnAlO₃ would denote the equilibrium regionbetween AlN, Er_(4-4x) Ho_(4x) Al₂ O₉ and Er_(1-x) Ho_(x) AlO₃.

For the case of Ln denoting Er+Ho, i.e. a mixture, the region involvingAlN and Ln₂ O₃ would denote the region involving AlN and Er₂ O₃ and Ho₂O₃. For example, 90 vol. % AlN and 10 vol. % Ln₂ O₃ would denote 90 vol.% AlN+ 10(1-x) vol. % Er₂ O₃ +10x vol. % Ho₂ O₃ where O<x<1. The regioninvolving AlN, Ln₄ Al₂ O₉ and LnAlO₃ would denote the region involvingAlN, Er₄ Al₂ O₉, Ho₄ Al₂ O₉, ErAlO₃ and HoAlO₃. For example, 94 vol. %AlN+5 vol. % Ln₄ Al₂ O₉ +1 vol. % LnAlO₃ would denote 94 vol. %AlN+5(1-x) vol. % Er₄ Al₂ O₉ +5 x vol. % Ho₄ Al₂ O₉ +(1-x) vol. % ErAlO₃+x vol. % HoAlO₃ where O<x<1.

As another example, 5 equivalent % Ln denotes 5 eq. % Er or 5 eq. % Hoor 5(1-x) eq. % Er+5 x eq. % Ho where O<x<1.

Briefly stated, the present process for producing the present sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon ABCDEFGH excluding line DE of FIGS. 1or 2, a porosity of less than 10% by volume, and a thermal conductivitygreater than 0.70 W/cm·K at 25° C. comprises the steps:

(a) forming a mixture comprised of aluminum nitride powder containingoxygen, an Ln oxide selected from the group consisting essentially oferbium oxide, holmium oxide, a mixture and solid solution thereof orprecursor therefor, and a carbonaceous additive selected from the groupconsisting essentially of free carbon, a carbonaceous organic materialand mixtures thereof, said carbonaceous organic material thermallydecomposing at a temperature ranging from about 50° C. to about 1000° C.to free carbon and gaseous product of decomposition which vaporizesaway, shaping said mixture into a compact, said mixture and said compacthaving a composition wherein the equivalent % of Ln and aluminum rangesfrom point A up to point E of FIG. 2, which is from greater than about0.25 equivalent % to about 8.0 equivalent % Ln and from less than about99.75 equivalent % to about 92.0 equivalent % aluminum, said mixture andsaid compact having an equivalent % composition of Ln, Al, O and Noutside the composition defined and encompassed by polygon ABCDEFGH ofFIG. 2,

(b) heating said compact in a nonoxidizing atmosphere at a temperatureup to about 1200° C. thereby providing erbium oxide and/or holmium oxideand free carbon,

(c) heating said compact in a nitrogen-containing nonoxidizingatmosphere at a temperature ranging from about 1350° C. to a temperaturesufficient to deoxidize the compact but below its pore closingtemperature reacting said free carbon with oxygen contained in saidaluminum nitride producing a deoxidized compact, said deoxidized compacthaving a composition wherein the equivalent % of Al, Ln, O and N isdefined and encompassed by polygon ABCDEFGH excluding line DE of FIG. 2,said free carbon being in an amount which produces said deoxidizedcompact, and

(d) sintering said deoxidized compact in a nitrogen-containingnonoxidizing atmosphere at a minimum temperature of about 1840° C.producing said polycrystalline body.

In the present process, the composition of the deoxidized compact inequivalent % is the same as or does not differ significantly from thatof the resulting sintered body in equivalent %.

In the present invention, oxygen content can be determined by neutronactivation analysis.

By weight % or % by weight of a component herein, it is meant that thetotal weight % of all the components is 100%.

By ambient pressure herein, it is meant atmospheric or about atmosphericpressure.

By specific surface area or surface area of a powder herein, it is meantthe specific surface area according to BET surface area measurement.

In another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point A to point F of FIG. 2, said Ln ranging from about 8.0equivalent % to about 0.3 equivalent %, said aluminum ranging from about92.0 equivalent % to about 99.7 equivalent %, said mixture and saidcompact having an equivalent % composition of Ln, Al, O and N outsidethe composition defined and encompassed by polygon ABCFGH of FIG. 2, andsaid sintered body and said deoxidized compact are comprised of acomposition wherein the equivalent percent of Al, Ln, O and N is definedand encompassed by polygon ABCFGH of FIG. 2.

In another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point A up to point Q of FIG. 2, i.e., said Ln ranging fromabout 8.0 equivalent % to greater than about 0.55 equivalent % and saidaluminum ranging from about 92.0 equivalent % to less than about 99.45equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined andencompassed by polygon ABCQRS of FIG. 2, and said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Ln, O and N is defined and encompassed by polygon ABCQRSexcluding line CQ of FIG. 2.

In another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point I up to point Q but excludes line KQ of FIG. 2, i.e.,said Ln ranging from about 5.75 equivalent % to greater than about 0.55equivalent % and said aluminum ranging from about 94.25 equivalent % toless than about 99.45 equivalent %, said mixture and said compact havingan equivalent % composition of Ln, Al, O and N outside the compositiondefined and encompassed by polygon IJKQRS of FIG. 2, and said sinteredbody and said deoxidized compact are comprised of a composition whereinthe equivalent percent of Al, Ln, O and N is defined and encompassed bypolygon IJKQRS but excluding line KQ of FIG. 2.

In yet another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point M up to point Q of FIG. 2, i.e., said Ln ranging fromabout 3.75 equivalent % to greater than about 0.55 equivalent % and saidaluminum ranging from about 96.25 equivalent % to less than about 99.45equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined andencompassed by polygon MNOQRS of FIG. 2, said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Ln, O and N is defined and encompassed by polygon MNOQRSexcluding line OQ of FIG. 2, and said minimum sintering temperature isabout 1850° C.

In another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point S up to point F of FIG. 2, i.e., said Ln ranging fromabout 2.4 equivalent % to greater than about 0.3 equivalent % and saidaluminum ranging from about 97.6 equivalent % to less than about 99.7equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N, outside the composition defined andencompassed by polygon SRQFGH of FIG. 2, said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Ln, O and N is defined and encompassed by polygon SRQFGH,excluding line QF of FIG. 2, and said minimum sintering temperatureranges from about 1890° C. to about 1920° C.

In yet another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point K to point O of FIG. 2, i.e., said Ln ranging fromabout 1.8 equivalent % to about 0.8 equivalent % and said aluminumranging from about 98.2 equivalent % to about 99.2 equivalent %, saidmixture and said compact having an equivalent % composition of Ln, Al, Oand N outside the composition defined by line KO of FIG. 2, and saidsintered body and said deoxidized compact are comprised of a compositionwherein the equivalent percent of Al, Ln, O and N is defined by line KOof FIG. 2.

In yet another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges from point O to point F of FIG. 2, i.e., said Ln ranging fromabout 0.8 equivalent % to about 0.3 equivalent % and said aluminumranging from about 99.2 equivalent % to about 99.7 equivalent %, saidmixture and said compact having an equivalent % composition of Ln, Al, Oand N outside the composition defined by line OF, said deoxidizedcompact and said sintered body are comprised of a composition whereinthe equivalent percent of Al, Ln, O and N is defined by line OF of FIG.2, and said minimum sintering temperature is about 1850° C.

In another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of Ln and aluminumranges between points K and E but does not include points K and E ofFIG. 2, said Ln ranging from less than about 1.8 equivalent % to greaterthan about 0.25 equivalent %, said aluminum ranging from greater thanabout 98.2 equivalent % to less than about 99.75 equivalent %, saidmixture and said compact having an equivalent % composition of Ln, Al, Oand N outside the composition defined by polygon KLEF of FIG. 2, andsaid deoxidized compact and said sintered body are comprised of acomposition wherein the equivalent percent of Al, Ln, O and N is definedand encompassed by polygon KLEF but does not include lines LE and KF ofFIG. 2.

The calculated equivalent % compositions of particular points in FIG. 2in the polygon ABCDEFGH representing compositions of particular sinteredbodies are shown in Tables I and II as well as their corresponding phasecomposition. Table I is based on erbium oxide in the present processwhereas Table II is based on holmium oxide. A comparison of Tables I andII shows that the equivalent % compositions are the same but that thevolume fraction of phases differs slightly at some points.

                                      TABLE I                                     __________________________________________________________________________    Composition                                                                   (Equivalent %)                                                                            Volume % of Phases                                                Point                                                                             Er Oxygen                                                                             AlN Er.sub.2 O.sub.3                                                                  Er.sub.4 Al.sub.2 O.sub.9                                                           ErAlO.sub.3                                                                        Er.sub.3 Al.sub.5 O.sub.12                     __________________________________________________________________________    A   8.0                                                                              8.0  86.7                                                                              13.3                                                                              --    --   --                                             B   5.8                                                                              9.0  86.9                                                                              --  13.1  --   --                                             C   2.6                                                                              6.0  93.8                                                                              --  --    6.2  --                                             D   1.3                                                                              4.7  95.5                                                                              --  --    --   4.5                                            E   0.25                                                                             1.85 99.1                                                                              --  --    --   0.9                                            F   0.3                                                                              1.4  99.3                                                                              --  --    0.7  --                                             G   0.35                                                                             0.9  99.2                                                                              --  0.8   --   --                                             H   1.9                                                                              1.9  96.7                                                                              3.3 --    --   --                                             I   5.75                                                                             5.75 90.3                                                                              9.7 --    --   --                                             J   4.2                                                                              6.7  90.3                                                                              --  9.7   --   --                                             K   1.8                                                                              4.3  95.8                                                                              --  --    4.2  --                                             L   0.8                                                                              3.4  97.2                                                                              --  --    --   2.8                                            M   3.75                                                                             3.75 93.6                                                                              6.4 --    --   --                                             N   2.4                                                                              4.0  94.4                                                                              --  5.6   --   --                                             O   0.8                                                                              2.4  98.0                                                                              --  --    2.0  --                                             P   0.5                                                                              2.5  98.3                                                                              --  --    --   1.7                                            Q   0.55                                                                             1.9  98.7                                                                              --  --    1.3  --                                             R   0.6                                                                              1.3  98.6                                                                              --  1.4   --   --                                             S   2.4                                                                              2.4  95.9                                                                              4.1 --    --   --                                             __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    Composition                                                                   (Equivalent %)                                                                            Volume % of Phases                                                Point                                                                             Ho Oxygen                                                                             AlN Ho.sub.2 O.sub.3                                                                  Ho.sub.4 Al.sub.2 O.sub.9                                                           HoAlO.sub.3                                                                        Ho.sub.3 Al.sub.5 O.sub.12                     __________________________________________________________________________    A   8.0                                                                              8.0  86.6                                                                              13.4                                                                              --    --   --                                             B   5.8                                                                              9.0  86.7                                                                              --  13.3  --   --                                             C   2.6                                                                              6.0  93.7                                                                              --  --    6.3  --                                             D   1.3                                                                              4.7  95.5                                                                              --  --    --   4.5                                            E   0.25                                                                             1.85 99.1                                                                              --  --    --   0.9                                            F   0.3                                                                              1.4  99.3                                                                              --  --    0.7  --                                             G   0.35                                                                             0.9  99.2                                                                              --  0.8   --   --                                             H   1.9                                                                              1.9  96.7                                                                              3.3 --    --   --                                             I   5.75                                                                             5.75 90.2                                                                              9.8 --    --   --                                             J   4.2                                                                              6.7  90.2                                                                              --  9.8   --   --                                             K   1.8                                                                              4.3  95.8                                                                              --  --    4.2  --                                             L   0.8                                                                              3.4  97.2                                                                              --  --    --   2.8                                            M   3.75                                                                             3.75 93.5                                                                              6.5 --    --   --                                             N   2.4                                                                              4.0  94.3                                                                              --  5.7   --   --                                             O   0.8                                                                              2.4  98.0                                                                              --  --    2.0  --                                             P   0.5                                                                              2.5  98.3                                                                              --  --    --   1.7                                            Q   0.55                                                                             1.9  98.7                                                                              --  --    1.3  --                                             R   0.6                                                                              1.3  98.6                                                                              --  1.4   --   --                                             S   2.4                                                                              2.4  95.8                                                                              4.2 --    --   --                                             __________________________________________________________________________

The polycrystalline aluminum nitride body produced by the presentprocess has a composition defined and encompassed by polygon ABCDEFGHexcluding line DE of FIG. 2. The sintered body of polygon ABCDEFGHexcluding line DE of FIG. 2 is comprised of from about 8.0 equivalent %to greater than about 0.25 equivalent % Ln, from about 92.0 equivalent %to less than about 99.75 equivalent % aluminum, from about 9.0equivalent % to about 0.9 equivalent % oxygen and from about 91.0equivalent % to about 99.1 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygonABCDEFGH excluding line DE of FIG. 2 is comprised of an AlN phase and asecond phase which ranges in amount from about 0.7% by volume LnAlO₃ atpoint F to about 13.4% by volume of Ln₂ O₃ at a composition at point A.As used herein, volume % or % by volume of a phase in the sintered bodymeans by volume of the sintered body. The composition of the sinteredbody in FIG. 2 varies as its position in the polygon is varied.

On lines AH, BG and CF, the sintered body is comprised of AlN and asecond phase. The sintered body on line AH has a phase compositioncomprised of AlN and Ln₂ O₃ with the amount of Ln₂ O₃ decreasing as thecomposition moves from point A to point H. On line BG, the compositionis comprised of AlN and Ln₄ Al₂ O₉ with the amount of Ln₄ Al₂ O₉decreasing as the composition moves from point B to point G. On line CF,the phase composition is comprised of AlN and LnAlO₃ with the amount ofLnAlO₃ decreasing as the composition moves from point C to point F.

In the area between lines AH and BG, the composition is comprised ofAlN, Ln₂ O₃ and Ln₄ Al₂ O₉ with the amount of Ln₂ O₃ decreasing as thecomposition moves away from line AH toward line BG and with the amountof Ln₄ Al₂ O₉ decreasing as the composition moves away from line BGtoward line AH. In the area between lines AH and BG, Ln₂ O₃ and Ln₄ Al₂O₉ are always present in at least a trace amount with Ln₂ O₃ ranging toa maximum amount of less than about 13.4 volume % and Ln₄ Al₂ O₉ rangingto a maximum amount of less than about 13.3 volume %.

In the area between lines BG and CF, the composition is comprised ofAlN, Ln₄ Al₂ O₉ and LnAlO₃ with the amount of Ln₄ Al₂ O₉ decreasing asthe composition moves away from line BG toward line CF and with theamount of LnAlO₃ decreasing as the composition moves away from line CFtoward line BG. In the area between lines BG and CF, Ln₄ Al₂ O₉ andLnAlO₃ are always present in at least a trace amount with Ln₄ Al₂ O₉ranging to a maximum amount of less than about 13.3 volume % and LnAlO₃ranging to a maximum amount of less than about 6.3 volume %.

In the area between lines CF and DE, the composition is comprised ofAlN, LnAlO₃ and Ln₃ Al₅ O₁₂ with the amount of LnAlO₃ decreasing as thecomposition moves away from line CF toward line DE and with the amountof Ln₃ Al₅ O₁₂ decreasing as the composition moves away from line DEtoward line CF. In the area between lines CF and DE, LnAlO₃ and Ln₃ Al₅O₁₂ are always present in at least a trace amount with LnAlO₃ ranging toa maximum amount of less than about 6.3 volume % and Ln₃ Al₅ O₁₂ rangingto a maximum amount of less than about 4.5 volume %.

The present polycrystalline sintered body having a composition definedand encompassed by polygon ABCDEFGH excluding line DE has a phasecomposition comprised of AlN and an Ln phase selected from the groupconsisting of from about 3.3% by volume to about 13.4% by volume of Ln₂O₃, from about 0.8% by volume to about 13.3% by volume of Ln₄ Al₂ O₉,from about 0.7% by volume to about 6.3% by volume of LnAlO₃, a mixtureof from a trace amount to less than about 13.4% by volume of Ln₂ O₃ andfrom a trace amount to less than about 13.3% by volume of Ln₄ Al₂ O₉, amixture of from a trace amount to less than about 13.3% by volume of Ln₄Al₂ O₉ and from a trace amount to less than about 6.3% by volume ofLnAlO₃, and a mixture of from a trace amount to less than about 6.3% byvolume of LnAlO₃ and from a trace amount to less than about 4.5% byvolume of Ln₃ Al₅ O₁₂.

By a trace or trace amount of a phase in the present sintered body, itis meant an amount detectable by X-ray diffraction analysis.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon ABCFGH of FIG. 2, i.e., it is comprised of fromabout 8.0 equivalent % to about 0.3 equivalent % Ln, from about 92.0equivalent % to about 99.7 equivalent % aluminum, from about 9.0equivalent % to about 0.9 equivalent % oxygen and from about 91.0equivalent % nitrogen to about 99.1 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygon ABCFGHof FIG. 2 is comprised of an AlN phase and an Ln phase selected from thegroup consisting of from about 3.3% by volume to about 13.4% by volumeof Ln₂ O₃, from about 0.8% by volume to about 13.3% by volume of Ln₄ Al₂O₉, from about 0.7% by volume to about 6.3% by volume of LnAlO₃, amixture of from a trace amount to less than about 13.4% by volume of Ln₂O₃ and from a trace amount to less than about 13.3% by volume Ln₄ Al₂O₉, and a mixture of from a trace amount to less than about 13.3% byvolume of Ln₄ Al₂ O₉ and from a trace amount to less than about 6.3% byvolume of LnAlO₃.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon ABCQRS excluding line CQ of FIG. 2, i.e. it iscomprised of from about 8.0 equivalent % to greater than about 0.55equivalent % Ln, from about 92.0 equivalent % to less than about 99.45equivalent % aluminum, from about 1.3 equivalent % to about 9.0equivalent % oxygen and from about 98.7 equivalent % to about 91.0equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygon ABCQRSexcluding line CQ of FIG. 2 is comprised of an AlN phase and an Ln phaseselected from the group consisting of from about 4.1% by volume to about13.4% by volume of Ln₂ O₃, from about 1.4% by volume to about 13.3% byvolume of Ln₄ Al₂ O₉, a mixture of from a trace to less than about 13.4%by volume of Ln₂ O₃ and from a trace to less than about 13.3% by volumeLn₄ Al₂ O₉, and a mixture of from a trace to less than about 13.3% byvolume of Ln₄ Al₂ O₉ and from a trace to less than about 6.3% by volumeof LnAlO₃.

In one embodiment, the present polycrystalline body has a compositiondefined and encompassed by polygon IJKQRS excluding line KQ of FIG. 2,i.e., it has a composition comprised of from about 5.75 equivalent % togreater than about 0.55 equivalent % Ln, from about 94.25 equivalent %to less than about 99.45 equivalent % aluminum, from about 6.7equivalent % to about 1.3 equivalent % oxygen and from about 93.3equivalent % to about 98.7 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygon IJKQRSexcluding line KQ of FIG. 2 has a phase composition comprised of AlN andan Ln phase selected from the group consisting of from about 4.1% byvolume to about 9.8% by volume of Ln₂ O₃, from about 1.4% by volume toabout 9.8% by volume of Ln₄ Al₂ O₉, a mixture of from a trace amount toless than about 9.8% by volume of Ln₂ O₃ and from a trace amount to lessthan about 9.8% by volume of Ln₄ Al₂ O₉, and a mixture of from a traceamount to less than about 9.8% by volume of Ln₄ Al₂ O₉ and from a traceto less than about 4.2% by volume of LnAlO₃.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon MNOQRS excluding line OQ of FIG. 2, i.e. it iscomprised of from about 3.75 equivalent % to greater than about 0.55equivalent % Ln, from about 96.25 equivalent % to less than about 99.45equivalent % aluminum, from about 4.0 equivalent % to about 1.3equivalent % oxygen and from about 96.0 equivalent % nitrogen to about98.7 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygon MNOQRSexcluding line OQ of FIG. 2 is comprised of an AlN phase and an Ln phaseselected from the group consisting of from about 4.1% by volume to about6.5% by volume of Ln₂ O₃, from about 1.4% by volume to about 5.7% byvolume of Ln₄ Al₂ O₉, a mixture of from a trace to less than about 6.5%by volume of Ln₂ O₃ and from a trace to less than about 5.7% by volumeof Ln₄ Al₂ O₉ and a mixture of from a trace to less than about 5.7% byvolume of Ln₄ Al₂ O₉ and from a trace to less than about 2.0% by volumeof LnAlO₃.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon SRQFGH excluding line QF of FIG. 2, i.e. it iscomprised of from about 2.4 equivalent % to greater than about 0.3equivalent % Ln, from about 97.6 equivalent % to less than about 99.7equivalent % aluminum, from about 2.4 equivalent % to about 0.9equivalent % oxygen and from about 99.1 equivalent % to about 97.6equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygon SRQFGHexcluding line QF of FIG. 2 is comprised of an AlN phase and an Ln phaseselected from the group consisting of from about 3.3% by volume to about4.2% by volume of Ln₂ O₃, from about 0.8% by volume to about 1.4% byvolume of Ln₄ Al₂ O₉, a mixture of from a trace to less than about 4.2%by volume of Ln₂ O₃ and from a trace to less than about 1.4% by volumeof L and a mixture of from a trace to less than about 1.4% by volume ofLn₄ Al₂ O₉ and from a trace to less than about 1.3% by volume of LnAlO₃.

In another embodiment, the present process produces a sintered bodydefined by line KO of FIG. 2, i.e. it is comprised of from about 1.8equivalent % to about 0.8 equivalent % Ln, from about 98.2 equivalent %to about 99.2 equivalent % aluminum, from about 4.3 equivalent % toabout 2.4 equivalent % oxygen and from about 95.7 equivalent % to about97.6 equivalent % nitrogen.

Line KO has a phase composition comprised of AlN and from about 2.0% byvolume to about 4.2% by volume of LnAlO₃.

In another embodiment, the present process produces a sintered bodydefined by line OF of FIG. 2, i.e. it is comprised of from about 0.8equivalent % to about 0.3 equivalent % Ln , from about 99.2 equivalent %to about 99.7 equivalent % aluminum, from about 2.4 equivalent % toabout 1.4 equivalent % oxygen and from about 97.6 equivalent % to about98.6 equivalent % nitrogen.

Line OF has a phase composition comprised of AlN and from about 0.7% byvolume to about 2.0% by volume of LnAlO₃.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon KLEF excluding lines LE and KF of FIG. 2, i.e.,it is comprised of from less than about 1.8 equivalent % to greater thanabout 0.25 equivalent % Ln, from greater than about 98.2 equivalent % toless than about 9.75 equivalent % aluminum, from less than about 4.3equivalent % to greater than about 1.4 equivalent % oxygen and fromgreater than about 95.7 equivalent % to less than about 98.6 equivalent% nitrogen.

Also, the polycrystalline body defined and encompassed by polygon KLEFexcluding lines LE and KF of FIG. 2 is comprised of an AlN phase and amixture of from a trace to less than about 4.2% by volume of LnAlO₃ andfrom a trace to less than about 2.8% by volume of Ln₃ Al₅ O₁₂.

In the present process, the aluminum nitride powder can be of commercialor technical grade. Specifically, it should not contain any impuritieswhich would have a significantly deleterious effect on the desiredproperties of the resulting sintered product. The starting aluminumnitride powder used in the present process contains oxygen generallyranging in amount up to about 4.4% by weight and usually ranging fromgreater than about 0.5% by weight to less than about 4.0% by weight,i.e. up to about 4% by weight, and more preferably ranging from greaterthan about 1.00% by weight to less than about 4.00% by weight.Typically, commercially available aluminum nitride powder contains fromabout 1.5 weight % (2.6 equivalent %) to about 3 weight % (5.2equivalent %) of oxygen and such powders are most preferred on the basisof their substantially lower cost.

Generally, the present starting aluminum nitride powder has a specificsurface area which can range widely, and generally it ranges up to about10 m² /g. Frequently, it has a specific surface area greater than about1.0 m² /g, and more frequently of at least about 3.0 m² /g, usuallygreater than about 3.2 m² /g, and preferably at least about 3.4 m² /g.

Generally, the present aluminum nitride powder in the present mixture,i.e. after the components have been mixed, usually by milling, has aspecific surface area which can range widely, and generally it ranges toabout 10 m² /g. Frequently, it ranges from greater than about 1.0 m² /gto about 10 m² /g, and more frequently from about 3.0 m² /g to about 10m² /g, and preferably it ranges from about 1.5 m² /g to about 5 m² /g,and more preferably it ranges from about 3.0 m² /g to about 5 m² /g,according to BET surface area measurement. Specifically, the minimumsintering temperature of a given composition of the present inventionincreases with increasing particle size of the aluminum nitride.

Generally, the lanthanide oxide (Ln₂ O₃) additive in the present mixturehas a specific surface area which can range widely. Generally, it isgreater than about 0.2 m² /g and generally it ranges from greater thanabout 0.2 m² /g to about 7.0 m² /g, usually from about 0.4 m² /g toabout 4.0 m² /g, and more usually from about 0.6 m² /g to about 3.5 m²/g.

In the practice of this invention, carbon for deoxidation of aluminumnitride powder is provided in the form of free carbon which can be addedto the mixture as elemental carbon, or in the form of a carbonaceousadditive, for example, an organic compound which can thermally decomposeto provide free carbon.

The present carbonaceous additive is selected from the group consistingof free carbon, a carbonaceous organic material and mixtures thereof.The carbonaceous organic material pyrolyzes, i.e. thermally decomposes,completely at a temperature ranging from about 50° C. to about 1000° C.to free carbon and gaseous product of decomposition which vaporizesaway. In a preferred embodiment, the carbonaceous additive is freecarbon, and preferably, it is graphite.

High molecular weight aromatic compounds or materials are the preferredcarbonaceous organic materials for making the present free carbonaddition since they ordinarily give on pyrolysis the required yield ofparticulate free carbon of submicron size. Examples of such aromaticmaterials are a phenolformaldehyde condensate resin known as Novolakwhich is soluble in acetone or higher alcohols, such as butyl alcohol,as well as many of the related condensation polymers or resins such asthose of resorcinol-formaldehyde, aniline-formaldehyde, andcresol-formaldehyde. Another satisfactory group of materials arederivatives of polynuclear aromatic hydrocarbons contained in coal tar,such as dibenzanthracene and chrysene. A preferred group are polymers ofaromatic hydrocarbons such as polyphenylene or polymethylphenylene whichare soluble in aromatic hydrocarbons.

The present free carbon has a specific surface area which can rangewidely and need only be at least sufficient to carry out the presentdeoxidation. Generally, it has a specific surface area greater thanabout 10 m² /g, preferably greater than 20 m² /g, more preferablygreater than about 100 m² /g, and still more preferably greater than 150m² /g, according to BET surface area measurement to insure intimatecontact with the AlN powder for carrying out its deoxidation.

Most preferably, the present free carbon has as high a surface area aspossible. Also, the finer the particle size of the free carbon, i.e. thehigher its surface area, the smaller are the holes or pores it leavesbehind in the deoxidized compact. Generally, the smaller the pores of agiven deoxidized compact, the lower is the amount of liquid phase whichneed be generated at sintering temperature to produce a sintered bodyhaving a porosity of less than about 1% by volume of the body.

By processing of the aluminum nitride powder into a compact fordeoxidation by free carbon, it is meant herein to include all mixing ofthe aluminum nitride powder to produce the present mixture, all shapingof the resulting mixture to produce the compact, as well as handling andstoring of the compact before it is deoxidized by carbon. In the presentprocess, processing of the aluminum nitride powder into a compact fordeoxidation by free carbon is at least partly carried out in air, andduring such processing of the aluminum nitride powder, it picks upoxygen from air usually in an amount greater than about 0.03% by weightof the aluminum nitride, and any such pick up of oxygen is controllableand reproducible or does not differ significantly if carried out underthe same conditions. If desired, the processing of the aluminum nitridepowder into a compact for deoxidation by free carbon can be carried outin air.

In the present processing of aluminum nitride, the oxygen it picks upcan be in any form, i.e. it initially may be oxygen, or initially it maybe in some other form, such as, for example, water. The total amount ofoxygen picked up by aluminum nitride from air or other media is lessthan about 3.00% by weight, and generally ranges from greater than about0.03% by weight to less than about 3.00% by weight, and usually itranges from about 0.10% by weight to about 1.00% by weight, andpreferably it ranges from about 0.15% by weight to about 0.70% byweight, of the total weight of the aluminum nitride. Generally, thealuminum nitride in the present mixture and compact prior to deoxidationof the compact has an oxygen content of less than about 4.70% by weight,and generally it ranges from greater than about 0.6% by weight,preferably greater than about 1.40% by weight, to less than about 4.70%by weight, and usually it ranges from about 2.00% by weight to about4.00% by weight, and more usually it ranges from about 2.20% by weightto about 3.50% by weight, of the total weight of aluminum nitride.

The oxygen content of the starting aluminum nitride powder and that ofthe aluminum nitride in the compact prior to deoxidation is determinableby neutron activation analysis.

In a compact, an aluminum nitride containing oxygen in an amount ofabout 4.7% by weight or more generally is not desirable.

In carrying out the present process, a uniform or at least asignificantly uniform mixture or dispersion of the aluminum nitridepowder, lanthanide oxide powder and carbonaceous additive, generally inthe form of free carbon powder, is formed and such mixture can be formedby a number of techniques. Preferably, the powders are ball milledpreferably in a liquid medium at ambient pressure and temperature toproduce a uniform or significantly uniform dispersion. The millingmedia, which usually are in the form of cylinders or balls, should haveno significant deleterious effect on the powders, and preferably, theyare comprised of steel or polycrystalline aluminum nitride, preferablymade by sintering a compact of milling media size of AlN powder and Ln₂O₃ sintering additive. Generally, the milling media has a diameter of atleast about 1/4 inch and usually ranges from about 1/4 inch to about 1/2inch in diameter. The liquid medium should have no significantlydeleterious effect on the powders and preferably it is non-aqueous.Preferably, the liquid mixing or milling medium can be evaporated awaycompletely at a temperature ranging from about room or ambienttemperature to below 300° C. leaving the present mixture. Preferably,the liquid mixing medium is an organic liquid such as heptane, hexane ortrichloroethane. Also, preferably, the liquid milling medium contains adispersant for the aluminum nitride powder thereby producing a uniformor significantly uniform mixture in a significantly shorter period ofmilling time. Such dispersant should be used in a dispersing amount andit should evaporate or decompose and evaporate away completely or leaveno significant residue, i.e. no residue which has a significant effectin the present process, at an elevated temperature below 1000° C.Generally, the amount of such dispersant ranges from about 0.1% byweight to less than about 3% by weight of the aluminum nitride powder,and generally it is an organic liquid, preferably oleic acid.

In using steel milling media, a residue of steel or iron is left in thedried dispersion or mixture which can range from a detectable amount upto about 3.0% by weight of the mixture. This residue of steel or iron inthe mixture has no significant effect in the present process or on thethermal conductivity of the resulting sintered body.

The liquid dispersion can be dried by a number of conventionaltechniques to remove or evaporate away the liquid and produce thepresent particulate mixture. If desired, drying can be carried out inair. Drying of a milled liquid dispersion in air causes the aluminumnitride to pick up oxygen and, when carried out under the sameconditions, such oxygen pick up is reproducible or does not differsignificantly. Also, if desired, the dispersion can be spray dried.

A solid carbonaceous organic material is preferably admixed in the formof a solution to coat the aluminum nitride particles. The solventpreferably is non-aqueous. The wet mixture can then be treated to removethe solvent producing the present mixture. The solvent can be removed bya number of techniques such as by evaporation or by freeze drying, i.e.subliming off the solvent in vacuum from the frozen dispersion. In thisway, a substantially uniform coating of the organic material on thealuminum nitride powder is obtained which on pyrolysis produces asubstantially uniform distribution of free carbon.

The present mixture is shaped into a compact in air, or includesexposing the aluminum nitride in the mixture to air. Shaping of thepresent mixture into a compact can be carried out by a number oftechniques such as extrusion, injection molding, die pressing, isostaticpressing, slip casting, roll compaction or forming, or tape casting toproduce the compact of desired shape. Any lubricants, binders or similarshaping aid materials used to aid shaping of the mixture should have nosignificant deteriorating effect on the compact or the present resultingsintered body. Such shaping-aid materials are preferably of the typewhich evaporate away on heating at relatively low temperatures,preferably below 400° C., leaving no significant residue. Preferably,after removal of the shaping aid materials, the compact has a porosityof less than 60% and more preferably less than 50% to promotedensification during sintering.

If the compact contains carbonaceous organic material as a source offree carbon, it is heated at a temperature ranging from about 50° C. toabout 1000° C. to pyrolyze, i.e. thermally decompose, the organicmaterial completely producing the present free carbon and gaseousproduct of decomposition which vaporizes away. Thermal decomposition ofthe carbonaceous organic material is carried out, preferably in a vacuumor at ambient pressure, in a nonoxidizing atmosphere. Preferably, thenonoxidizing atmosphere in which thermal decomposition is carried out isselected from the group consisting of nitrogen, hydrogen, a noble gassuch as argon and mixtures thereof, and more preferably it is nitrogen,or a mixture of at least about 25% by volume nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon andmixtures thereof. In one embodiment, it is a mixture of nitrogen andfrom about 1% by volume to about 5% by volume hydrogen.

The actual amount of free carbon introduced by pyrolysis of thecarbonaceous organic material can be determined by pyrolyzing theorganic material alone and determining weight loss. Preferably, thermaldecomposition of the organic material in the present compact is done inthe sintering furnace as the temperature is being raised to deoxidizingtemperature, i.e. the temperature at which the resulting free carbonreacts with the oxygen content of the AlN.

Alternately, in the present process, the lanthanide oxide can beprovided by means of a precursor therefor. The precursor can be anyorganic or inorganic compound which decomposes completely at an elevatedtemperature below about 1200° C. to form the present lanthanide oxideand by-product gas which vaporizes away leaving no contaminants in thesintered body which would be detrimental to the thermal conductivity.Representative of the precursors of the present lanthanide oxide usefulin the present process is the acetate, carbonate, oxalate, nitrate,sulfate and hydroxide of erbium or holmium.

If the compact contains a precursor for the lanthanide oxide, it isheated to a temperature up to about 1200° C. to thermally decompose theprecursor thereby providing the lanthanide oxide. Such thermaldecomposition is carried out in a nonoxidizing atmosphere, preferably ina vacuum or at ambient pressure, and preferably the atmosphere isselected from the group consisting of nitrogen, hydrogen, a noble gassuch as argon and mixtures thereof. Preferably, it is nitrogen, or amixture of at least about 25% by volume nitrogen and a gas selected fromthe group consisting of hydrogen, a noble gas such as argon and mixturesthereof. In one embodiment, it is a mixture of nitrogen and from about1% by volume to about 5% by volume hydrogen.

The present deoxidation of aluminum nitride with carbon, i.e.carbon-deoxidation, comprises heating the compact comprised of aluminumnitride, free carbon and lanthanide oxide at deoxidation temperature toreact the free carbon with at least a sufficient amount of the oxygencontained in the aluminum nitride to produce the present deoxidizedcompact. This deoxidation with carbon is carried out at a temperatureranging from about 1350° C. to a temperature at which the pores of thecompact remain open, i.e. a temperature which is sufficient to deoxidizethe compact but below its pore closing temperature, generally up toabout 1800° C., and preferably, it is carried out at from about 1600° C.to 1650° C.

The carbon-deoxidation is carried out, preferably at ambient pressure,in a gaseous nitrogen-containing nonoxidizing atmosphere which containssufficient nitrogen to facilitate the deoxidation of the aluminumnitride. In accordance with the present invention, nitrogen is arequired component for carrying out the deoxidation of the compact.Preferably, the nitrogen-containing atmosphere is nitrogen, or it is amixture of at least about 25% by volume of nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon, andmixtures thereof. Also, preferably, the nitrogen-containing atmosphereis comprised of a mixture of nitrogen and hydrogen, especially a mixturecontaining up to about 5% by volume hydrogen.

The time required to carry out the present carbon-deoxidation of thecompact is determinable empirically and depends largely on the thicknessof the compact as well as the amount of free carbon it contains, i.e.the carbon-deoxidation time increases with increasing thickness of thecompact and with increasing amounts of free carbon contained in thecompact. Carbon-deoxidation can be carried out as the compact is beingheated to sintering temperature provided that the heating rate allowsthe deoxidation to be completed while the pores of the compact are openand such heating rate is determinable empirically. Also, to some extent,carbon deoxidation time depends on deoxidation temperature, particlesize and uniformity of the particulate mixture of the compact i.e. thehigher the deoxidation temperature, the smaller the particle size andthe more uniform the mixture, the shorter is deoxidation time. Also, tosome extent, deoxidation time depends on its final position on the phasediagram, i.e. as line AH is approached, deoxidation time increases.Typically, the carbon-deoxidation time ranges from about 1/4 hour toabout 1.5 hours.

Preferably, the compact is deoxidized in the sintering furnace byholding the compact at deoxidation temperature for the required time andthen raising the temperature to sintering temperature. The deoxidationof the compact must be completed before sintering closes off pores inthe compact preventing gaseous product from vaporizing away and therebypreventing production of the present sintered body.

In the present deoxidation with carbon, the free carbon reacts with theoxygen of the aluminum nitride producing carbon monoxide gas whichvaporizes away. It is believed that the following deoxidation reactionoccurs wherein the oxygen content of the aluminum nitride is given asAl₂ O₃ :

    Al.sub.2 O.sub.3 +3C+N.sub.2 →3CO.sub.(g) +2AlN     (2)

In the deoxidation effected by carbon, gaseous carbon-containing productis produced which vaporizes away thereby removing free carbon.

lf the compact before deoxidation is heated at too fast a rate throughthe carbon-deoxidation temperature to sintering temperature, and suchtoo fast rate would depend largely on the composition of the compact andthe amount of carbon it contains, the present carbon-deoxidation doesnot occur i.e. an insufficient amount of deoxidation occurs, and asignificant amount of carbon is lost by reactions (3) and/or (3A).

    C+AlN→AlCN.sub.(g)                                  (3)

    C+1/2N.sub.2 →CN.sub.(g)                            (3A)

The specific amount of free carbon required to produce the presentdeoxidized compact can be determined by a number of techniques. It canbe determined empirically. Preferably, an initial approximate amount ofcarbon is calculated from Equation (2), that is the stoichiometricamount for carbon set forth in Equation (2), and using such approximateamount, the amount of carbon required in the present process to producethe present sintered body would require one or a few runs to determineif too much or too little carbon had been added. Specifically, this canbe done by determining the porosity of the sintered body and byanalyzing it for carbon and by X-ray diffraction analysis. If thecompact contains too much carbon, the resulting deoxidized compact willbe difficult to sinter and will not produce the present sintered bodyhaving a porosity of less than about 10% by volume, or the sintered bodywill contain carbon in an excessive amount. If the compact contains toolittle carbon, X-ray diffraction analysis of the resulting sintered bodywill show that it does not have the present composition.

The amount of free carbon used to carry out the present deoxidationshould produce the present deoxidized compact leaving no significantamount of carbon in any form, i.e. no amount of carbon in any form whichwould have a significantly deleterious effect on the sintered body. Morespecifically, no amount of carbon in any form should be left in thedeoxidized compact which would prevent production of the presentsintered body, i.e. any carbon content in the sintered body should below enough so that the sintered body has a thermal conductivity greaterthan 0.70 W/cm·K at 25° C. Generally, the present sintered body maycontain carbon in some form in a trace amount, i.e. generally less thanabout 0.08% by weight, preferably in an amount of less than about 0.065%by weight, and more preferably less than about 0.04% by weight, and mostpreferably less than 0.03% by weight of the total weight of the sinteredbody.

A significant amount of carbon in any form remaining in the sinteredbody significantly reduces its thermal conductivity. An amount of carbonin any form greater than about 0.065% by weight of the sintered body islikely to significantly decrease its thermal conductivity.

The present deoxidized compact is densified, i.e. liquid-phase sintered,at a temperature which is a sintering temperature for the composition ofthe deoxidized compact to produce the present sintered body. For thepresent deoxidized compact, this sintering temperature generally is atleast about 1840° C. and generally ranges from about 1840° C. to about2050° C. with the minimum sintering temperature increasing generallyfrom about 1840° C. for a composition at point to generally about 1960°C. for a composition at point A to about 1980° C. at point H of FIG. 2.Minimum sintering temperature is dependent most strongly on compositionand less strongly on particle size.

Specifically, the minimum sintering temperature is dependent largely onthe composition (i.e., position in the FIG. 2 phase diagram), the greendensity of the compact, i.e. the porosity of the compact after removalof shaping aid materials but before deoxidation, the particle size ofaluminum nitride, and to a much lesser extent the particle size oflanthanide oxide and carbon. The minimum sintering temperature increasesas the composition moves away from line BCD toward line AH and alsotoward line HGFE. In addition, it increases as the green density of thecompact decreases, and as the particle size of aluminum nitride, and toa much lesser extent, lanthanide oxide and carbon increases.

To carry out the present liquid phase sintering, the present deoxidizedcompact contains sufficient equivalent percent of Ln and O to form asufficient amount of liquid phase at sintering temperature to densifythe carbon deoxidized compact to produce the present sintered body. Thepresent minimum densification, i.e. sintering, temperature depends onthe composition of the deoxidized compact, i.e. the amount of liquidphase it generates. Specifically, for a sintering temperature to beoperable in the present invention, it must generate at least sufficientliquid phase in the particular composition of the deoxidized compact tocarry out the present liquid phase sintering to produce the presentproduct. For a given composition, the lower the sintering temperature,the smaller is the amount of liquid phase generated, i.e. densificationbecomes more difficult with decreasing sintering temperature. However, asintering temperature higher than about generally 2050° C. provides nosignificant advantage.

The deoxidized compact is sintered, preferably at or about ambientpressure, in a gaseous nitrogen-containing nonoxidizing atmosphere whichcontains at least sufficient nitrogen to prevent significant weight lossof aluminum nitride. In accordance with the present invention, nitrogenis a necessary component of the sintering atmosphere to prevent anysignificant weight loss of AlN during sintering, and also to optimizethe deoxidation treatment and to remove carbon. Significant weight lossof the aluminum nitride can vary depending on its surface area to volumeratio, i.e. depending on the form of the body, for example, whether itis in the form of a thin or thick tape. As a result, generally,significant weight loss of aluminum nitride ranges from in excess ofabout 5% by weight to in excess of about 10% by weight of the aluminumnitride. Preferably, the nitrogen-containing atmosphere is nitrogen, orit is a mixture at least about 25% by volume nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon andmixtures thereof. Also, preferably, the nitrogen-containing atmosphereis comprised of a mixture of nitrogen and hydrogen, especially a mixturecontaining from about 1% by volume to about 5% by volume hydrogen.

Sintering time is determinable empirically. Typically, sintering timeranges from about 40 minutes to about 90 minutes.

The present sintered polycrystalline body is a pressureless sinteredceramic body. By pressureless sintering herein it is meant thedensification or consolidation of the deoxidized compact without theapplication of mechanical pressure into a ceramic body having a porosityof less than about by volume, and preferably less than about 4% byvolume.

The polycrystalline body of the present invention is liquid-phasesintered. I.e., it sinters due to the presence of a liquid phase, thatis liquid at the sintering temperature and is rich in Ln and oxygen andcontains some aluminum and nitrogen. In the present polycrystallinebody, the AlN grains have about the same dimensions in all directions,and are not elongated or disk shaped. Generally, the AlN in the presentpolycrystalline body has an average grain size ranging from about 1micron to about 20 microns. An intergranular second phase of Ln₂ O₃ orLn₄ Al₂ O₉ or LnAlO₃ or a mixture of some of the following phases: Ln₂O₃, Ln₄ Al₂ O₉, LnAlO₃, Ln₃ Al₅ O₁₂ is present along some of the AlNgrain boundaries. The morphology of the microstructure of the presentsintered body indicates that this intergranular second phase or phaseswas a liquid at the sintering temperature. As the composition approachesline AB in FIG. 2, the amount of liquid phase increases and the AlNgrains in the present sintered body become more rounded and have asmoother surface. As the composition moves away from line AB in FIG. 2and approaches line EG, the amount of liquid phase decreases and the AlNgrains in the present sintered body become less rounded and the cornersof the grains become sharper.

The present sintered body has a porosity of less than about 10% byvolume, and generally less than about 4% by volume, of the sinteredbody. Preferably, the present sintered body has a porosity of less thanabout 2% and most preferably less than about 1% by volume of thesintered body. Any pores in the sintered body are fine sized, andgenerally they are less than about 1 micron in diameter. Porosity can bedetermined by standard metallographic procedures and by standard densitymeasurements.

The present process is a control process for producing a sintered bodyof aluminum nitride having a thermal conductivity greater than 0.70W/cm·K at 25° C. Generally, the thermal conductivity of the presentpolycrystalline body is less than that of a high purity single crystalof aluminum nitride which is about 2.8 W/cm·K at 25° C. If the sameprocedure and conditions are used throughout the present process, theresulting sintered body has a thermal conductivity and composition whichis reproducible or does not differ significantly. Generally, in a giventwo-phase region or three-phase region thermal conductivity increaseswith a decrease in volume % of second phase, and for a given compositionwith increase in sintering temperature.

In the present process, aluminum nitride picks up oxygen in acontrollable or substantially controllable manner. Specifically, if thesame procedure and conditions are used in the present process, theamount of oxygen picked up by aluminum nitride is reproducible or doesnot differ significantly. Also, erbium oxide and holmium oxide do notpick up oxygen, or do not pick up any significant amount of oxygen, fromair or other media in the present process. More specifically, in thepresent process, erbium oxide or holmium oxide or the present precursortherefor do not pick up any amount of oxygen in any form from the air orother media which would have any significant effect on thecontrollability or reproducibility of the present process. Any oxygenwhich erbium oxide or holmium oxide might pick up in the present processis so small as to have no effect or no significant effect on the thermalconductivity or composition of the resulting sintered body.

Examples of calculations for equivalent % are as follows:

For a starting aluminum nitride powder weighing 89.0 grams measured ashaving 2.3 weight % oxygen, it is assumed that all of the oxygen isbound to AlN as Al₂ O₃, and that the measured 2.3 weight % of oxygen ispresent as 4.89 weight % Al₂ O₃ so that the AlN powder is assumed to becomprised of 84.65 grams AlN and 4.35 grams Al₂ O₃.

A mixture is formed comprised of 89.0 grams of the starting AlN powder,8.00 grams of Er₂ O₃ and 1.4 grams free carbon.

During processing, this AlN powder picks up additional oxygen byreactions similar to (4) and now contains 2.6 weight % oxygen.

    2 AlN+3H.sub.2 O→Al.sub.2 O.sub.3 +2NH.sub.3        (4)

The resulting compact now is comprised of the following composition:

89.11 grams AlN powder containing 2.6 weight % oxygen, (84.19 gAlN+4.92g Al₂ O₃), 8.00 grams Er₂ O₃ and 1.4 grams carbon.

During deoxidation of the compact, all the carbon is assumed to reactwith Al₂ O₃ via reaction (5)

    Al.sub.2 O.sub.3 +3C+N.sub.2 →2AlN+3CO.sub.(g)      (5)

In the present invention, the carbon will not reduce Er₂ O₃, butinstead, reduces Al₂ O₃.

After reaction (5) has gone to completion, the deoxidized compact now iscomprised of the following composition which was calculated on the basisof Reaction (5):

88.34 grams AlN powder containing 0.5 weight % oxygen 87.38 gramsAlN+0.96 grams Al₂ O₃) and 8.00 grams Er₂ O₃

From this weight composition, the composition in equivalent % can becalculated as follows:

    ______________________________________                                        Wt (g)         Moles        Equivalents                                       ______________________________________                                        AlN     87.38      2.132        6.395                                         Al.sub.2 O.sub.3                                                                      0.958      9.40  × 10.sup.-3                                                                    5.640 × 10.sup.-2                       Er.sub.2 O.sub.3                                                                      8.00       2.090 × 10.sup.-2                                                                    0.125                                         ______________________________________                                        TOTAL EQUIVALENTS = 6.577                                                     V = Valence                                                                    ##STR1##                                                                     MW = molecular weight                                                         Eq = Equivalents                                                              Eq = M × V                                                              Valences:                                                                     Al + 3                                                                        Er + 3                                                                        N - 3                                                                         O - 2                                                                         Eq % Er in deoxidized compact =                                                ##STR2##                      (6)                                             ##STR3##                                                                     Eq % O in deoxidized compact =                                                 ##STR4##                      (7)                                             ##STR5##                      (8)                                            This deoxidized compact as well as the sintered body contains about 1.91  

To produce the present sintered body containing 2.0 equivalent % Er and2.8 equivalent % O, i.e. comprised of 2 equivalent % Er, 98 equivalent %Al, 2.8 equivalent % O and 7.2 equivalent % N, using an AlN powdermeasured as having 2.3 weight % Oxygen (4.89 weight % Al₂ O₃), thefollowing calculations for weight % from equivalent % can be made:

100 grams=weight of AlN powder

x grams=weight of Er₂ O₃ powder

z grams=weight of Carbon powder

Assume that during processing, the AlN powder picks up additional oxygenby reaction similar to (9) and in the compact before deoxidation nowcontains 2.6 weight % oxygen (5.52 weight % Al₂ O₃) and weighs 100.12grams

    2AlN+3H.sub.2 O→Al.sub.2 O.sub.3 +2NH.sub.3         (9)

After processing, the compact can be considered as having the followingcomposition:

    ______________________________________                                        Weight (g)      Moles       Equivalents                                       ______________________________________                                        AlN    94.59        2.308       6.923                                         Al.sub.2 O.sub.3                                                                      5.53        0.0542      0.325                                         Er.sub.2 O.sub.3                                                                     x            2.614 × 10.sup.-3 x                                                                   0.01569 x                                   C      z            .0833 z                                                   ______________________________________                                    

During deoxidation, 3 moles of carbon reduce 1 mole of Al₂ O₃ and in thepresence of N₂ form 2 moles of AlN by the reaction:

    Al.sub.2 O.sub.3 +3C+N.sub.2 →2AlN+3CO              (10)

After deoxidation, all the carbon will have reacted and the compact canbe considered as having the following composition:

    ______________________________________                                        Weight (g)     Moles        Equivalents                                       ______________________________________                                        AlN   94.59 + 2.275z                                                                             2.308  + 0.05551z                                                                          6.923 + 0.1665z                               Al.sub.2 O.sub.3                                                                     5.53 - 2.830z                                                                             0.0542 - 0.02775z                                                                          0.325 - 0.1665z                               Er.sub.2 O.sub.3                                                                    x            2.614 × 10.sup.-3 x                                                                  0.01569x                                      ______________________________________                                        T = Total Equivalents = 7.248 + 0.01569x                                       ##STR6##                     (11)                                            Equivalent Fraction of O =                                                     ##STR7##                     (12)                                            Solving Equations (11) and (12) for x and z:

x=9.43 grams of Er₂ O₃ powder

z=1.60 grams of free carbon

A body in a form or shape useful as a substrate, i.e. in the form of aflat thin piece of uniform thickness, or having no significantdifference in its thickness, usually referred to as a substrate or tape,may become non-flat, for example, warp, during sintering and theresulting sintered body may require a heat treatment after sintering toflatten it out and make it useful as a substrate. This non-flatness orwarping is likely to occur in the sintering of a body in the form of asubstrate or tape having a thickness of less than about 0.070 inch andcan be eliminated by a flattening treatment, i.e. by heating thesintered body, i.e. substrate or tape, under a sufficient appliedpressure at a temperature in the present sintering temperature range offrom about 1840° C. to about 2050° C. for a period of time determinableempirically, and allowing the sandwiched body to cool to below itssintering temperature, preferably to ambient or room temperature, beforerecovering the resulting flat substrate or tape.

Specifically, in one embodiment of this flattening process, the non-flatsubstrate or tape is sandwiched between two plates and may be separatedfrom such plates by a thin layer of AlN powder depending largely on itscomposition and the applied pressure. The sandwiched body is heated toits sintering temperature, i.e. a temperature which is a sinteringtemperature for the sandwiched sintered body, preferably in the sameatmosphere used for sintering, under an applied pressure at leastsufficient to flatten the body, generally at least about 0.03 psi, for atime period sufficient to flatten the sandwiched body, and then thesandwiched body is allowed to cool to below its sintering temperaturebefore it is recovered.

One embodiment for carrying out this flattening treatment of a sinteredthin body or substrate tape comprises sandwiching the sintered non-flatsubstrate or tape between two plates of a material which has nosignificant deleterious effect thereon such as molybdenum or tungsten,or an alloy containing at least about 80% by weight of tungsten ormolybdenum. The sandwiched substrate or tape is separated from theplates by a thin layer, preferably a discontinuous coating, preferably adiscontinuous monolayer, of aluminum nitride powder preferably justsufficient to prevent the body from sticking to the surfaces of theplates during the flattening heat treatment. The flattening pressure isdeterminable empirically and depends largely on the particular sinteredbody, the particular flattening temperature and flattening time period.The flattening treatment should have no significant deleterious effecton the sintered body. A decrease in flattening temperature requires anincrease in flattening pressure or flattening time. Generally, at atemperature ranging from about 1840° C. to about 2050° C., the appliedflattening pressure ranges from about 0.03 psi to about 1.0 psi,preferably from about 0.06 psi to about 0.50 psi. Typically, forexample, heating the sandwiched sintered body at the sinteringtemperature under a pressure of from about 0.03 psi to about 0.5 psi for1 hour in nitrogen produces a flat body useful as a substrate,especially as a supporting substrate for a semiconductor such as asilicon chip.

The present invention makes it possible to fabricate simple, complexand/or hollow shaped polycrystalline aluminum nitride ceramic articlesdirectly. Specifically, the present sintered body can be produced in theform of a useful shaped article without machining or without anysignificant machining, such as a hollow shaped article for use as acontainer, a crucible, a thin walled tube, a long rod, a spherical body,a tape, substrate or carrier. It is useful as a sheath for temperaturesensors. It is especially useful as a substrate for a semiconductor suchas a silicon chip. The dimensions of the present sintered body differfrom those of the unsintered body, by the extent of shrinkage, i.e.densification, which occurs during sintering.

The present ceramic body has a number of uses. In the form of a thinflat piece of uniform thickness, or having no significant difference inits thickness, i.e. in the form of a substrate or tape, it is especiallyuseful as packaging for integrated circuits and as a substrate for anintegrated circuit, particularly as a substrate for a semiconducting Sichip for use in computers.

The invention is further illustrated by the following examples whereinthe procedure was as follows, unless otherwise stated:

The starting aluminum nitride powder was greater than 99% pure AlNexclusive of oxygen.

The starting aluminum nitride powder had a surface area of 3.4 m² /g(0.54 micron) and based on a series of deoxidations carried out withcarbon powder, it was determined to have contained about 2.4 wt %oxygen.

The Er₂ O₃ powder, before any mixing, i.e., as received, had a surfacearea of 2.4 m² /g.

The carbon was graphite and it had a specific surface area of 200 m² /g(0.017 micron) as listed by the vendor.

In all of the examples, the milling media was hot-pressed aluminumnitride in the approximate form of cubes or rectangles having a densityof about 100%.

In all of the examples, the dried milled powder mixture was die pressedat the given pressure in air at room temperature to produce a compacthaving a density greater than 50% of its theoretical density.

The compacts were in the form of square rods.

In all of the examples, the given powder mixture as well as the compactformed therefrom had a composition wherein the equivalent % of erbiumand aluminum ranged between points A and E of FIG. 2.

The equivalent % composition of Er, Al, O and N of the compacts of allof the examples, i.e., before deoxidation, was outside the compositiondefined and encompassed by polygon ABCDEFGH of FIG. 2.

In all of the examples, the aluminum nitride in the compact beforedeoxidation contained oxygen in an amount of less than about 4.70% byweight of the aluminum nitride.

The composition of the deoxidized compacts of all of the examples wasdefined and encompassed by polygon ABCDEFGH of FIG. 2 excluding line DE.

In each example, the same atmosphere was used to carry out thedeoxidation of the compact as was used to carry out the sintering of thedeoxidized compact. The atmosphere to carry out the deoxidization wasfed into the furnace at a rate of 1 SCFH to promote removal of the gasesproduced by deoxidation, and the flow rate during sintering was lessthan about 0.1 SCFH.

The atmosphere during all of the heat treatment in the examples was atambient pressure which was atmospheric or about atmospheric pressure.

The furnace was a molybdenum heat element furnace.

The compacts were heated in the furnace to about 800° C. at about 25°C./min and then to the given deoxidation temperature at the rate ofabout 100° C. per minute and then to the given sintering temperature atthe rate of about 50° C. per minute.

Based on other experiments, it was estimated that the aluminum nitridein the compact before deoxidation had an oxygen content of about 0.15%by weight to about 0.70% by weight higher than that of the startingaluminum nitride powder.

In Table III, the equivalent % composition of the sintered body wascalculated from the starting powder composition and from the X-raydiffraction phase analysis of the sintered body. The Er, Al, N andoxygen are assumed to have their conventional valences of: +3, +3, -3,-2, respectively. In the sintered bodies, the equivalent percent amountof Er and Al was assumed to be the same as that in the starting powder.The equivalent percent oxygen and nitrogen are approximate and weredetermined from the equivalent percent erbium and Al and the X-raydiffraction phase analyses of the sintered bodies.

Density of the sintered body was determined by the Archimedes method.

Porosity in % by volume of the sintered body was determined by knowingthe theoretical density of the sintered body on the basis of itscomposition and comparing that to the density measured using thefollowing equation: ##EQU2##

The phase composition of each sintered body was determined by X-raydiffraction analysis.

The thermal conductivity of the sintered body of Example 1 was measuredat 25° C. by a steady state heat-flow method using a rodshaped sample˜0.4 cm×0.4 cm×2.2 cm sectioned from the sintered body. This method wasoriginally devised by A. Berget in 1888 and is described in an articleby G. A. Slack in the "Encyclopaedic Dictionary of Physics", Ed. by J.Thewlis, Pergamon, Oxford, 1961. In this technique the sample is placedinside a high-vacuum chamber, heat is supplied at one end by anelectrical heater, and the temperatures are measured with fine-wirethermocouples. The sample is surrounded by a guard cylinder. Theabsolute accuracy is about ±3% and the repeatability is about ±1%. As acomparison, the thermal conductivity of an Al₂ O₃ single crystal wasmeasured with a similar apparatus to be 0.44 W/cm·K at about 22° C.

EXAMPLE 1

4.328 grams of Er₂ O₃ powder and 0.361 grams of graphite powder wereadded to 30.11 grams of aluminum nitride powder and the mixture, alongwith aluminum nitride milling media, was immersed in trichloroethanecontaining oleic acid in an amount of about 0.7% by weight of thealuminum nitride powder in a plastic jar and vibratory milled in theclosed jar at room temperature for about 22 hours. The milling media wasthen removed and a binder solution was added to the milled dispersionand the mixture was then roll mixed for about 4 hours. The bindersolution was comprised of 1.13 grams of an organic binder, i.e.,polyvinyl butyral which was in the form of a white powder, 0.43 grams ofa plasticizer, i.e., dioctylphthalate, and 0.17 grams of a lubricant,i.e., polyethylene propylene oxide, which were dissolved intrichloroethane. The resulting dispersion was dried in air for about 2-3hours and during such drying, the aluminum nitride picked up oxygen fromthe air. During milling, the mixture picked up 0.50 grams of AlN due towear of the AlN milling media.

A portion of the resulting dried mixture was die pressed producing acompact.

The compact was placed on a molybdenum plate and heated in an atmospherecomprised of nitrogen and 2% hydrogen to 1600° C. where it was held for1 hour, and then the temperature was raised to 1950° C. where it washeld for 1 hour, and then the sample was furnace cooled to ambienttemperature.

This example is shown as Example 1 in Table III. The sintered bodyproduced in Example 1 had a phase composition comprised of AlN and Er₄Al₂ O₉ in an amount of about 6.9% by volume of the sintered body. Also,it had an equivalent % composition comprised of about 4.8% O, (100%-4.8)or 95.2% N, 2.9% Er and (100%-2.9%) or 97.1% Al.

Example 2 was carried out in substantially the same manner as Example 1except as indicated herein and except as shown in Table III.

                                      TABLE III                                   __________________________________________________________________________                         Heat Treatment                                           Powder Mixture  Pressing                                                                           Deoxidation                                                                            + Sintering                                     (wt %)          Pressure                                                                           Temp                                                                              - Time Temp                                                                              - Time                                                                             - Atmosphere                         Ex                                                                              Sample                                                                            AlN                                                                              Er.sub.2 O.sub.3                                                                  C  KPSI (°C.)                                                                      - (Hr) (°C.)                                                                      - (Hr)                                    __________________________________________________________________________    1 212A                                                                              86.72                                                                            12.26                                                                             1.02                                                                             20   1600                                                                              - 1  + 1950                                                                              - 1  - N.sub.2 + 2% H.sub.2               2 220A                                                                              93.96                                                                             4.80                                                                             1.24                                                                             15   1600                                                                              - 1  + 1850                                                                              - 1  - N.sub.2 + 2%                       __________________________________________________________________________                                               H.sub.2                                          Properties of Sintered Body                                                                  Approximate                                                    Equivalent %                                                                            Density                                                                            Porosity                                                                             Volume % of Second Phases                             Ex                                                                              Oxygen                                                                             Erbium                                                                             (g/cc)                                                                             (vol. %)                                                                             Er.sub.2 O.sub.3                                                                  Er.sub.4 Al.sub.2 O.sub.9                                                          ErAlO.sub.3                      __________________________________________________________________________                1 ˜4.8                                                                         2.9  3.53 <1     --  6.9  --                                           2 ˜2.7                                                                         1.1  3.37 <2     0.3 --   2.2                              __________________________________________________________________________

Examples 1 and 2 illustrate the present invention. The sintered bodiesproduced in Examples 1 and 2 were about 0.16 in. by 0.16 in. by 1.5 in.Based on other work, it was known that the sintered bodies produced inExamples 1 and 2 had a carbon content of less than about 0.065% byweight. The thermal conductivity of the sintered body produced inExample 1 was measured and found to be 1.36 W/cm·K at 25° C.

Based on other experiments and a comparison of Examples 1 and 2, it wasknown that the sintered body produced in Example 2 had a thermalconductivity greater than 0.7 W/cm·K at 25° C.

The sample produced in Example 2 contained second phases of ErAlO₃ and asmall amount of Er₂ O₃. Had the sample reached equilibrium duringsintering, the resulting sintered sample would have contained secondphases of Er₄ Al₂ O₉ and ErAlO₃, and would not contain any Er₂ O₃.Apparently, due to the low sintering temperature, the small amount ofEr₂ O₃ did not chemically react to form Er₄ Al₂ O₉. However, the overallsample equivalent % composition lies within polygon JKQR.

The sintered bodies produced in Examples 1 and 2 would be useful forpackaging of integrated circuits as well as for use as a substrate orcarrier for a semiconductor such as a silicon chip.

What is claimed is:
 1. A process for producing a liquid phase sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon ABCDEFGH excluding line DE of FIG. 2,a porosity of less than about 10% by volume of said body and a thermalconductivity greater than 0.70 W/cm·K at 25° C. which consistsessentially of the steps:(a) forming a mixture consisting essentially ofan oxygen-containing aluminum nitride powder, a lanthanide oxideselected from the group consisting of erbium oxide, holmium oxide, amixture and solid solution thereof, and free carbon, shaping saidmixture into a compact, said mixture and said compact having acomposition wherein the equivalent % of said lanthanide and aluminumranges from point A up to point E of FIG. 2, said lanthanide rangingfrom greater than about 0.25 equivalent % to about 8.0 equivalent %,said aluminum ranging from less than about 99.75 equivalent % to about92.0 equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined andencompassed by polygon ABCDEFGH of FIG. 2, (b) heating said compact in anitrogen-containing nonoxidizing atmosphere at a temperature rangingfrom about 1350° C. to a temperature sufficient to deoxidize the compactbut below its pore closing temperature reacting said free carbon withoxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Ln, O and N is defined and encompassed by polygonABCDEFGH excluding line DE of FIG. 2, said free carbon being in anamount which produces said deoxidized compact, an (c) sintering saiddeoxidized compact in a nitrogen-containing nonoxidizing atmosphere at aminimum temperature of about 1840° C. producing said polycrystallinebody.
 2. The process according to claim 1 wherein saidnitrogen-containing atmosphere in step (b) contains sufficient nitrogento facilitate deoxidation of the aluminum nitride to produce saidsintered body.
 3. The process according to claim 1 wherein saidnitrogen-containing atmosphere in step (c) contains sufficient nitrogento prevent significant weight loss of said aluminum nitride.
 4. Theprocess according to claim 1 wherein said process is carried out atambient pressure.
 5. The process according to claim 1 wherein saidporosity is less than about 4% by volume of said body.
 6. The processaccording to claim 1 wherein said mixture and said compact have acomposition wherein the equivalent % of said lanthanide and aluminumranges from point A to point F of FIG. 2, said lanthanide ranging fromabout 8.0 equivalent % to about 0.3 equivalent %, said aluminum rangingfrom about 92.0 equivalent % to about 99.7 equivalent %, said mixtureand said compact having an equivalent % composition of Ln. Al, O and Noutside the composition defined and encompassed by polygon ABCFGH ofFIG. 2, and wherein said sintered body and said deoxidized compact arecomprised of a composition wherein the equivalent percent of Al, Ln, Oand N is defined and encompassed by polygon ABCFGH of FIG.
 2. 7. Theprocess according to claim 1 wherein said mixture and said compact havea composition wherein the equivalent % of said lanthanide and aluminumranges from point A up to point Q of FIG. 2, said lanthanide rangingfrom about 8.0 equivalent % to greater than about 0.55 equivalent % andsaid aluminum ranging from about 92.0 equivalent % to less than about99.45 equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined and IOencompassed by polygon ABCQRS of FIG. 2, and said deoxidized compact andsintered body are comprised of a composition wherein the equivalentpercent of Al, Ln, O and N is defined and encompassed by polygon ABCQRSexcluding line CQ of FIG.
 2. 8. The process according to claim 1 whereinsaid mixture and said compact have a composition wherein the equivalent% of said lanthanide and aluminum ranges from point I up to point Q ofFIG. 2, said lanthanide ranging from about 5.75 equivalent % to greaterthan about 0.55 equivalent % and said aluminum ranging from about 94.25equivalent % to less than about 99.45 equivalent %, said mixture andsaid compact having an equivalent % composition of Ln, Al, O and Noutside the composition defined and encompassed by polygon IJKQRS ofFIG. 2, and said deoxidized compact and sintered body are comprised of acomposition wherein the equivalent % of Al, Ln, O and N is defined andencompassed by polygon IJKQRS but excluding line KQ of FIG.
 2. 9. Theprocess according to claim 1 wherein said mixture and said compact havea composition wherein the equivalent % of said lanthanide and aluminumranges from point M up to point Q of FIG. 2, said lanthanide rangingfrom about 3.75 equivalent % to greater than about 0.55 equivalent % andsaid aluminum ranging from about 96.25 equivalent % to less than about99.45 equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined andencompassed by polygon MNOQRS of FIG. 2, said deoxidized compact andsintered body are comprised of a composition wherein the equivalent % ofAl, Ln, O and N is defined and encompassed by polygon MNOQRS excludingline OQ of FIG. 2, and wherein said minimum sintering temperature isabout 1850° C.
 10. The process according to claim 1 wherein said mixtureand said compact have a composition wherein the equivalent % of saidlanthanide and aluminum ranges from point S up to point F of FIG. 2,said lanthanide ranging from about 2.4 equivalent % to greater thanabout 0.3 equivalent % and said aluminum ranging from about 97.6equivalent % to less than about 99.7 equivalent %, said mixture and saidcompact having an equivalent % composition of Ln, Al, O and N outsidethe composition defined and encompassed by polygon SRQFGH of FIG. 2,said deoxidized compact and sintered body are comprised of a compositionwherein the equivalent percent of Al, Ln, O and N is defined andencompassed by polygon SRQFGH, excluding line QF of FIG. 2, and whereinsaid minimum sintering temperature ranges from about 1890° C. to about1920° C.
 11. The process according to claim 1 wherein said mixture andsaid compact have a composition wherein the equivalent % of saidlanthanide and aluminum ranges from point K to point O of FIG. 2, saidlanthanide ranging from about 1.8 equivalent % to about 0.8 equivalent %and said aluminum ranging from about 98.2 equivalent % to about 99.2equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined by lineKO of FIG. 2, and said deoxidized compact and sintered body arecomprised of a composition wherein the equivalent percent of Al, Ln, Oand N is defined by line KO of FIG.
 2. 12. The process according toclaim 1 wherein said mixture and said compact have a composition whereinthe equivalent % of said lanthanide and aluminum ranges from point O topoint F of FIG. 2, said Ln ranging from about 0.8 equivalent % to about0.3 equivalent % and said aluminum ranging from about 99.2 equivalent %to about 99.7 equivalent %, said mixture and said compact having anequivalent % composition of Ln, Al, O and N outside the compositiondefined by line OF, said deoxidized compact and said sintered body arecomprised of a composition wherein the equivalent percent of Al, Ln, Oand N is defined by line OF of FIG. 2, and wherein said minimumsintering temperature is about 1850° C.
 13. The process according toclaim 1 wherein said mixture and said compact have a composition whereinthe equivalent % of said lanthanide and aluminum ranges between points Kand E but does not include points K and E of FIG. 2, said lanthanideranging from less than about 1.8 equivalent % to greater than about 0.25equivalent %, said aluminum ranging from greater than about 98.2equivalent % to less than about 99.75 equivalent %, said mixture andsaid compact having an equivalent % composition of Ln, Al, O and Noutside the composition defined by polygon KLEF of FIG. 2, and saiddeoxidized compact and said sintered body are comprised of a compositionwherein the equivalent percent of Al, Ln, O and N is defined andencompassed by polygon KLEF but does not include lines LE and KF of FIG.2.
 14. A process for producing a sintered polycrystalline aluminumnitride ceramic body having a composition defined and encompassed bypolygon ABCDEFGH excluding line DE of FIG. 2, a porosity of less thanabout 10% by volume of said body and a thermal conductivity greater than0.70 W/cm· at 25° C. which consists essentially the steps:(a) forming amixture consisting essentially of an oxygen-containing aluminum nitridepowder, a lanthanide oxide selected from the group consisting of erbiumoxide, holmium oxide, a mixture and solid solution thereof or precursortherefor, and a carbonaceous additive selected from the group consistingessentially of free carbon, a carbonaceous organic material and mixturethereof, said carbonaceous organic material thermally decomposing at atemperature ranging from about 50° C. to about 1000° C. to free carbonand gaseous product of decomposition which vaporizes away, shaping saidmixture into a compact, said mixture and said compact having acomposition wherein the equivalent % of said lanthanide and aluminumranges from point A up to point E of FIG. 2, said lanthanide rangingfrom greater than about 0.25 equivalent % to about 8.0 equivalent %,said aluminum ranging from less than about 99.75 equivalent % to about92.0 equivalent % aluminum, said mixture and said compact having anequivalent % composition of Ln, Al, O and N outside the compositiondefined and encompassed by polygon ABCDEFGH of FIG. 2, (b) heating saidcompact in a nonoxidizing atmosphe,re at a temperature up to about 1200°C. thereby providing lanthanide oxide and free carbon, (c) heating saidcompact in a nitrogen-containing nonoxidizing atmosphere at atemperature ranging from about 1350° C. to a temperature sufficient todeoxidize the compact but below its pore closing temperature reactingsaid free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Ln, O and N is defined andencompassed by polygon ABCDEFGH excluding line DE of FIG. 2, said freecarbon being in an amount which produces said deoxidized compact, and(d) sintering said deoxidized compact in a nitrogen-containingnonoxidizing atmosphere at a minimum temperature of about 1840° C.producing said polycrystalline body.
 15. The process according to claim14 wherein said nitrogen-containing atmosphere in step (c) containssufficient nitrogen to facilitate deoxidation of the aluminum nitride toproduce said sintered body.
 16. The process according to claim 14wherein said nitrogen-containing atmosphere in step (d) containssufficient nitrogen to prevent significant weight loss of said aluminumnitride.
 17. The process according to claim 14 wherein said process iscarried out at ambient pressure.
 18. The process according to claim 14wherein said porosity is less than about 4% by volume of said body. 19.The process according to claim 14 wherein said mixture and said compacthave a composition wherein the equivalent % of said lanthanide andaluminum ranges from point A to point F of FIG. 2, said lanthanideranging from about 8.0 equivalent % to about 0.3 equivalent %, saidaluminum ranging from about 92.0 equivalent % to about 99.7 equivalent%, said mixture and said compact having an equivalent % composition ofLn, Al, O and N outside the composition defined and encompassed bypolygon ABCFGH of FIG. 2, and wherein said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Ln, O and N is defined by polygon ABCFGH of FIG.
 2. 20.The process according to claim 14 wherein said mixture and said compacthave a composition wherein the equivalent % of said lanthanide andaluminum ranges from point A up to point Q of FIG. 2, said lanthanideranging from about 8.0 equivalent % to greater than about 0.55equivalent % and said aluminum ranging from about 92.0 equivalent % toless than about 99.45 equivalent %, said mixture and said compact havingan equivalent % composition of Ln, Al, O and N outside the compositiondefined and encompassed by polygon ABCQRS of FIG. 2, and said deoxidizedcompact and sintered body are comprised of a composition wherein theequivalent percent of Al, Ln, O and N is defined and encompassed bypolygon ABCQRS excluding line CQ of FIG.
 2. 21. The process according toclaim 14 wherein said mixture and said compact have a compositionwherein the equivalent % of said lanthanide and aluminum ranges frompoint I up to point Q but excludes line KQ of FIG. 2, said lanthanideranging from about 5.75 equivalent % to greater than about 0.55equivalent % and said aluminum ranging from about 4.25 equivalent % toless than about 99.45 equivalent %, said mixture and said compact havingan equivalent % composition of Ln, Al, O and N outside the compositiondefined and encompassed by polygon IJKQRS of FIG. 2, and said deoxidizedcompact and sintered body are comprised of a composition wherein theequivalent percent of Al, Ln, O and N is defined and encompassed bypolygon IJKQRS but excluding line KQ of FIG.
 2. 22. The processaccording to claim 14 wherein said mixture and said compact have acomposition wherein the equivalent % of said lanthanide and aluminumranges from point M up to point Q of FIG. 2, said lanthanide rangingfrom about 3.75 equivalent % to greater than about 0.55 equivalent % andsaid aluminum ranging from about 96.25 equivalent % to less than about99.45 equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined andencompassed by polygon MNOQRS of FIG. 2, said deoxidized compact andsintered body are comprised of a composition wherein the equivalent % ofAl, Ln, O and N is defined and encompassed by polygon MNOQRS excludingline OQ of FIG. 2, and wherein said minimum sintering temperature isabout 1850° C.
 23. The process according to claim 14 wherein saidmixture and said compact have a composition wherein the equivalent % ofsaid lanthanide and aluminum ranges from point S up to point F of FIG.2, said lanthanide ranging from about 2.4 equivalent % to greater thanabout 0.3 equivalent % and said aluminum ranging from about 97.6equivalent % to less than about 99.7 equivalent %, said mixture and saidcompact having an equivalent % composition of Ln, Al, O and N, outsidethe composition defined and encompassed by polygon SRQFGH of FIG. 2,said deoxidized compact and sintered body are comprised of a compositionwherein the equivalent percent of Al, Ln, O and N is defined andencompassed by polygon SRQFGH excluding line QF of FIG. 2, and whereinsaid minimum sintering temperature ranges from about 1890° C. to about1920° C.
 24. The process according to claim 14 wherein said mixture andsaid compact have a composition wherein the equivalent % of saidlanthanide and aluminum ranges from point K to point O of FIG. 2, saidlanthanide ranging from about 1.8 equivalent % to about 0.8 equivalent %and said aluminum ranging from about 98.2 equivalent % to about 99.2equivalent %, said mixture and said compact having an equivalent %composition of Ln, Al, O and N outside the composition defined by lineKO of FIG. 2, and said deoxidized compact and sintered body arecomprised of a composition wherein the equivalent percent of Al, Ln, Oand N is defined by line KO of FIG.
 2. 25. The process according toclaim 14 wherein said mixture and said compact have a compositionwherein the equivalent % of said lanthanide and aluminum ranges frompoint O to point F of FIG. 2, said Ln ranging from about 0.8 equivalent% to about 0.3 equivalent % and said aluminum ranging from about 99.2equivalent % to about 99.7 equivalent %, said mixture and said compacthaving an equivalent % composition of Ln, Al, O and N outside thecomposition defined by line OF, said deoxidized compact and saidsintered body are comprised of a composition wherein the equivalentpercent of Al, Ln, O and N is defined by line OF of FIG. 2, and whereinsaid minimum sintering temperature is about 1850° C.
 26. The processaccording to claim 14 wherein said mixture and said compact have acomposition wherein the equivalent % of said lanthanide and aluminumranges between points K and E but does not include points K and E ofFIG. 2, said lanthanide ranging from less than about 1.8 equivalent % togreater than about 0.25 equivalent %, said aluminum ranging from greaterthan about 98.2 equivalent % to less than about 99.75 equivalent %, saidmixture and said compact having an equivalent % composition of Ln, Al, Oand N outside the composition defined by polygon KLEF of FIG. 2, andsaid deoxidized compact and said sintered body are comprised of acomposition wherein the equivalent percent of Al, Ln, O and N is definedand encompassed by polygon KLEF but does not include lines LE and KF ofFIG. 2.